Rationalize Dart mojo and sky package structure

sky/framework/elements/material-element.sky

-<!--
-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
--->
-<import src="sky-element.sky" />
-<import src="sky-ink-splash.sky" />
-<script>
-import "dart:sky";
-
-HTMLStyleElement _kStyleElement;
-
-class MaterialElement extends SkyElement {
-  Set<SkyInkSplash> _activeSplashes = new Set();
-  bool _didAddTransformHack = false;
-
-  MaterialElement() {
-    addEventListener('pointerdown', _handlePointerDown);
-    addEventListener('gesturescrollstart', _handleScrollStart);
-  }
-
-  void addTransformHack() {
-    if (_didAddTransformHack)
-      return;
-    if (style['transform'] != null)
-      return;
-    _didAddTransformHack = true;
-    // We set the transform here to become a container for absolutely positioned
-    // elements. We should either have a better way of becoming such a container
-    // or we should make every RenderBlock be such a container.
-    style['transform'] = 'translateX(0)';
-  }
-
-  void removeTransformHack() {
-    if (!_didAddTransformHack)
-      return;
-    style.removeProperty('transform');
-    _didAddTransformHack = false;
-  }
-
-  void _handlePointerDown(PointerEvent event) {
-    addTransformHack();
-    ClientRect rect = getBoundingClientRect();
-    SkyInkSplash splash = new SkyInkSplash();
-    shadowRoot.prependChild(splash);
-    splash.start(event.x, event.y, rect).then(_handleSplashComplete);
-    _activeSplashes.add(splash);
-  }
-
-  void _handleSplashComplete(SkyInkSplash splash) {
-    _activeSplashes.remove(splash);
-    if (_activeSplashes.isEmpty)
-      removeTransformHack();
-  }
-
-  void _handleScrollStart(GestureEvent event) {
-    for (SkyInkSplash splash in _activeSplashes) {
-      splash.cancel();
-    }
-  }
-}
-</script>